After the Deluge: Dam! We Almost Lost a Dam!

But for the vagaries of the weather, we would never have seen it. Running field studies courses in the winter and spring always involves an element of chance, but in the dry year that we've had, it seemed safe enough to schedule a trip to Yosemite National Park in early April. But as the previous two posts have shown, an epic atmospheric river storm, a Pineapple Express, pummeled the area last Saturday. We postponed our trip to Sunday, and had a fine day, but we reversed the direction of our trip, because we still weren't sure if Yosemite Valley would open up in time for us to visit. If the valley remained closed, we wanted the choice of going to Hetch Hetchy Valley instead, and to allow that we needed to go by way of Highway 120 through Big Oak Flat and Groveland, instead of Highway 140 through Mariposa.

I had heard that Moccasin Reservoir almost failed a few weeks earlier, but didn't give it much thought while I was bustling about trying to think of alternate field trip stops. We had left the Red Hills Area of Critical Environmental Concern and were approaching the Priest Grade when I remembered the dam episode. During an intense storm on March 22, floodwaters overwhelmed the spillway of the small earthen dam on Moccasin Creek. Moccasin Dam is about 700 feet across and about 60 feet high, holding back about 554 acre-feet of water when full.

There is one very strange fact about the dam. Even though it blocks Moccasin Creek, it doesn't hold water from Moccasin Creek! In instead contains Tuolumne River water that has been diverted into the Hetch Hetchy system and pumped through a hydroelectric plant on lower Moccasin Creek. The dam is a small forebay that feeds into the much larger Don Pedro Reservoir and Lake just a mile downstream. Moccasin Creek was heavily mined during the Gold Rush, and the sediments may be contaminated by mercury and other toxic metals related to the gold extraction process. The water of Moccasin Creek is diverted around the reservoir.

As we drove by, I commented on the radios that the flood had happened, but I saw that a lot of the damage was visible from the highway and we screeched to a stop to have a look. It had clearly been a serious storm. The flood had ripped out trees, roads, and telephone poles. The dam itself looked undamaged, but the spillway downstream had been severely eroded. Few people were threatened by the flooding, but roads were closed. If the dam had failed, the effects would have been limited because Don Pedro Lake, only a mile downstream, would have easily absorbed the extra water.

Maybe the saddest aspect of the flood was the near destruction of the fish hatchery. As can be seen in the picture below by Mark Brooks and aired on several news outlets, the entire hatchery was flooded, and all of the fish either killed by turbulent mud, or carried downstream to Don Pedro (when does fishing season start?).

Geology and weather are capricious beasts. One of the lessons of the earth sciences is that no place anywhere on this planet is free of the possibility of natural disasters. Some places are more dangerous than others, but there is always a big advantage to understanding the possible hazards where one lives and works. One may think they may never happen in one's lifetime, and they might not, but that's a dangerous gamble to make.

Photo by Mark Brooks